First caisson near Venice successfully immersed

11 June 2014

The first caisson of the Venetian storm-surge barrier was immersed successfully on Saturday, 7 June 2014, in the Adriatic Sea near Venice. The next of the eight elements will follow on Friday 20 June. Strukton Immersion Projects is responsible for floating the caissons to the surface, transporting them and then immersing them again at their final locations. Preparations have been underway for more than a year.

Preparations

On Friday 6 June, the 40-man team worked on the last preparations for immersion. Immersion Expert Peter van Westendorp explains: “The caissons have been ready for some time now. They're lying in the construction dock until we're ready for them. A few days ago, the first caisson was floated to the surface and pulled to its temporary mooring location, where it was prepared for immersion; this involved checking all systems, sailing the immersion pontoon over the caisson and securing the lifting points. The caisson was also baptized Gabry-Sarah with a bottle of prosecco, in line with the Strukton Immersion Projects' tradition to name caissons before they are immersed.

Accurate to the millimetre

The caisson is to be positioned to within a tolerance of 10 millimetres at a depth of 23 metres. Peter van Westendorp comments: “This positioning must be performed in conditions that aren't always smooth sailing. Waves up to 0.75 metres and a flow rate of up to 1.25 metres per second are the conditions that need to be taken into account. The total system being used to immerse the caisson is made up of 14 winches that should ensure that the element is placed in the correct location. The accuracy needed and the conditions present make the positioning operation very complicated and risky. In addition, all of the diving must be performed in a relatively strong current, and this also makes the immersion process difficult."

The immersion process

The first caisson (weighing in on 21,000,000 kilograms) was towed from the construction dock to the immersion location early Saturday morning 7 June at 3:00 AM. As the connection of the tugboats to the caisson went quickly and the tide was favorable, the caisson moved to the navigation channel ahead of schedule. The caisson was then moved in position for instalment in the immersion trench and the winches that hold the caisson and the immersion pontoon in position were attached. After that, the pumps that ballast the caisson were installed.

Immersing the caisson with a minimal roll and pitch was a challenge due to the sheer size of the caisson (24 meters high) and the fact that the caisson is compartmentalized into 14 differently sized compartments. Fortunately, the efforts of the engineering department resulted in a controlled immersion of the caisson into the Adriatic Sea. Around 2:00 AM the caisson touched down and was accepted into its final position at 11:00 AM on 8 June.